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Am I Under-firing my Furnace?

Last month I installed a GMV series 95%, 2-stage Goodman furnace in my own home. The manifold pressure the Goodman specs recommend are a range (Low: 1.6-2.2, High: 3.2-3.8). The temp rise for each stage is specified as 30-60F. I set the manifold pressure to the minimum recommended. I have a temp rise of about 35F for both stages. Clocking the gas meter, I get inputs of 39K BTU and 58K BTU. This furnace is specified as a 70K BTU input furnace. I realize that I am within the specs for manifold pressure and temp rise, but I still worry that I might be under fired. I don't want to have any primary heat exchanger condensation happening.

Can any of you guys give me any reassurance to leave the furnace as it is, or reasons to increase the firing rate?

Ideal is 6% O2 and 120 to 140 flue temp. This will give you the best efficiency and longevity.

Why would you set the gas pressure to minimum??? That makes the furnace less efficient, especially in low fire. You will get the most btu's for your gas dollars by firing at the manufacturer's specified input rate.

That heat exchanger is notorious for rotting out right at the tail end of the primary HX. I believe it is mostly due to not having sufficient post purge after the burners shut off, but underfiring definitely accelerates the corrosion.

I did set it at the mfg specified input rate, well, at least I set the manifold pressure to the minimum published in the installation materials. The furnace is slightly oversized and I like it to run as long as possible. I don't really understand why this would make the furnace less efficient? I understand there are some duct losses, but I don't expect an efficiency loss from the furnace itself. So you are saying I should increase the manifold pressure until I am firing at near 70K BTU on high fire (so long as I don't go above the maximum published manifold pressure for my furnace)?

I will check the flue temp just for fun tomorrow. I believe it to easily be over 110F, at least right where it leaves the furnace. The leaving air from the furnace is 110 right now, and the return air is 68, so that is a 42 degree rise.

Perfectionist hit the nail on the head. The only way to know exactly what pressure to set gas at is to use CA. There are many types and all work a little different from one to the other, but the main goal of all is the same. It will sample the gases in the flew and give you a read out. The read out will help you to determine if the fuel to air mixture is correct. It can also be helpful to find venting issues, ( I.e. can not get right fuel air mixture next step investigate flew pipe setup.). Sad part is that everyone just sets the gas pressures up to within the recommended ranges: 90% of us dont use an anilizer to fine tune the system (Myself included, despite my efforts for the boss to get me one. I can't afford a $3,000.00 tool.) One last thing to keep in mind when fine tuning things is that if the furnace is in a small space (like a closet) have the door to this space closed, like in actual running conditions, to get a proper reading. Testing with a wide open door will give different results than testing within an enclosed space. This will allow you to see if you have proper makeup air. Also wouldn't be a bad idea to run water heater at the same time in this situation that is if it is gas fired.

Perfectionist hit the nail on the head. The only way to know exactly what pressure to set gas at is to use CA. There are many types and all work a little different from one to the other, but the main goal of all is the same. It will sample the gases in the flew and give you a read out. The read out will help you to determine if the fuel to air mixture is correct. It can also be helpful to find venting issues, ( I.e. can not get right fuel air mixture next step investigate flew pipe setup.). Sad part is that everyone just sets the gas pressures up to within the recommended ranges: 90% of us dont use an anilizer to fine tune the system (Myself included, despite my efforts for the boss to get me one. I can't afford a $3,000.00 tool.) One last thing to keep in mind when fine tuning things is that if the furnace is in a small space (like a closet) have the door to this space closed, like in actual running conditions, to get a proper reading. Testing with a wide open door will give different results than testing within an enclosed space. This will allow you to see if you have proper makeup air. Also wouldn't be a bad idea to run water heater at the same time in this situation that is if it is gas fired.

$3000?? You can get a very good analyzer for less than a third of that...

why do you want a furnace that blows colder air and takes longer to heat your house.

Actually, yes! I like long run cycles. Also, the registers are in the ceiling, so I like the discharge air to be cooler to help keep stratification to a minimum. There is hardly any perceptible temp swing now. We have been using the new furnace for a month and it is LOADS more comfortable that my old furnace with it's 150 degree discharge air.

I did set it at the mfg specified input rate, well, at least I set the manifold pressure to the minimum published in the installation materials. The furnace is slightly oversized and I like it to run as long as possible. I don't really understand why this would make the furnace less efficient? I understand there are some duct losses, but I don't expect an efficiency loss from the furnace itself. So you are saying I should increase the manifold pressure until I am firing at near 70K BTU on high fire (so long as I don't go above the maximum published manifold pressure for my furnace)?

I will check the flue temp just for fun tomorrow. I believe it to easily be over 110F, at least right where it leaves the furnace. The leaving air from the furnace is 110 right now, and the return air is 68, so that is a 42 degree rise.

It is easy enough to measure the efficiency (and I'm not talking about the efficiency reading on a combustion analyzer), this will show you how much less efficient it is firing at such a low firing rate. Heat exchangers are not variable size and are the most efficient at the maximum firing rate.